For many years, food products such as fresh poultry were shipped in bulk from the food processors to supermarkets where the bulk pieces were divided into small quantities and repackaged for retailing. For example, fresh poultry was cut up, placed on cardboard or plastic trays, and covered with stretch film secured to the tray by tack welding.
For improved efficiency the current trend is to perform the retail packaging operation in the food processing plant and ship the small retail packages from such plants to the retailers. There is also a trend towards evacuating and hermetically sealing the food retail packages in the central food processing plants, due to the longer storage time between retail packaging and consumer use. Such packaging increases the shelf life of the food packages. Also, there is a need for increased abuse resistant food retail packages because of the more frequent handling, impact and abrasion inherent in the above-described food processing plant retail packaging system.
The most commonly used film in the trayed food overwrap market is polyvinyl chloride (PVC). This thermoplastic polymer has been used because of satisfactory elongation and elastic memory. The elastic PVC contracts as the trayed food loses moisture and the food itself contracts. In some instances the result is a tight package which becomes leaky and thereby unattractive. This is because the PVC material in overwrapped trays is tack welded to itself, and not hermetically sealed. As the PVC contracts, leaks may develop between the tack welds. As a consequence, fluids which exude from overwrapped food (e.g. meats), may emerge from the package. This problem cannot be solved by hermetically sealing because of a very narrow sealing temperature range between which the seal is effective (at the low temperature end) and "burn through" or melting of the PVC (at the high temperature end). This range is so limited as to be impractical with many commercially employed heat sealing systems.
Another limitation of PVC tray overwrap material is its poor resistance to physical abuse. The PVC material tends to tear along the edges of the overwrapped tray if rubbed during transit by another tray or an enclosing carton.
One characteristic of PVC is that although it contracts, it is not "heat-shrinkable", i.e. it is not a material which will tend to return to its original unstretched (unextended) dimension when heated to its softening point. The terms "orientation" or "oriented" are used to describe the manufacture of heat-shrinkable films, wherein resin material is heated to its flow or melting point and extruded through a die into either tubular or sheet form. After cooling, the relatively thick extrudate is reheated to a temperature range suitable to orient or align the crystallites and/or molecules of the material. The orientation temperature range for a given material or materials is understood by those skilled in the art to be in a range which revises the inter-molecular configuration of the material by physical alignment of the crystallites and/or molecules of the material to improve certain mechanical properties of the film such as shrink tension, as for example measured in accordance with ASTM D-2838-81. When the stretching force is applied in one direction, uniaxial orientation results. When the stretching force is simultaneously applied in two directions, biaxial orientation results.
In view of the aforementioned limitations of PVC as a stretch film food tray overwrap material, there have been prior efforts to identify a heat-shrinkable thermoplastic film having an improved combination of elongation, elastic memory, heat sealability and puncture resistance. However, most heat-shrinkable thermoplastic film packaging materials suitable for food contact have relatively poor elasticity or elastic memory. Thus, when a food wrapped in such a material shrinks from moisture loss, the film does not shrink and a loose package results which is asthetically unpleasing.
One heat-shrinkable biaxially oriented film which has been commercially employed in the trayed food overwrap market is a five layer film designated by its manufacturer, W. R. Grace & Co.--Conn. as SSD-310. This polyolefin-containing composition provides many of the aforedescribed characteristics, but when used to hermetically seal the food-containing tray after evacuation, the power consumption of the impulse and heat sealing equipment is relatively high. This is in part because the SSD-310, due to its particular construction, must be irradiatively cross-linked for satisfactory bubble stability during biaxial orientation, and also for satisfactory abuse resistance. Another reason for the high power consumption is that the SSD-310 film requires a high sealing temperature on the order of 350.degree.-430.degree. F., which in turn requires a long dwell time.
Another important film property for food tray overwrap applications is low permanent deformation, which is a measure of the film's increase in length after stretching and relaxation. More particularly, the permanent deformation test used herein is derived from ASTM 621 and measures the percent increase in length of a film sample stretched 50% and then allowed to relax for 30 seconds. Lower percentage values reflect the ability of a film to relax after being stretched. In tray overwrap applications, good film recovery preserves package appearance and integrity following handling abuse. Perfect elastomers would have 0% permanent deformation. Plasticized PVC has 3-5% deformation, while polyolefins such as LLDPE generally have significantly higher permanent deformation, on the order of 6-15%.
One object of this invention is to provide an improved heat-shrinkable oriented stretch film suitable for use as a trayed food overwrap material.
Another object of this invention is to provide such a film which is capable of forming a hermetically sealed trayed food package as a stretch overwrap material.
Still another object is to provide such a film having high abuse resistance.
A further object is to provide such a heat-shrinkable oriented stretch film in the form of five or less multiple layers.
A still further object is to provide a stretch film which can be biaxially oriented without irradiative cross-linking.
An additional object is to provide such a film which can be hermetically sealed as a stretch wrap material around a food-containing tray at lower temperature and with less energy than heretofore required.
Another object is to provide a polyolefin-containing heat-shrinkable oriented multiple layer stretch film having low permanent deformation.
A still additional object of this invention is to provide an improved evacuated, hermetically sealed food-containing tray package overwrapped by a heat shrunk film.
Another object is an improved method for forming an evacuated, hermetically sealed high abuse resistant tray food package with a heat shrunk plastic film overwrap.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.